Medical containers that comprise a sealing stopper in a gliding engagement within a container body are widely used to deliver drug by injection to a patient.
Injection devices usually comprise such container which is intended to receive the drug to be injected and a plunger rod intended to move the stopper within the body of the container so as to expel the drug therefrom at the time of injection.
Either it is necessary to use an empty disposable injection device to withdraw the drug to be injected from a vial or to use disposable prefilled injection device.
Prefilled injection devices are often preferred because they are more convenient, safe and efficient and may reduce risk of cross contamination during preparation of the injection.
Many different types of injection devices, including syringes, cartridges and auto-injectors have been designed for administering drugs.
Infusion pumps are known to be used for delivering or dispensing drugs—such as insulin in case of diabetes—in a more comfortable way for the patient.
Such pumps comprise a pump housing adapted to receive an injection device containing the prescribed drug and an associated infusion set.
They usually include a small drive motor connected to a plunger rod for motor-driven advancement of the stopper present in the container in order to administer the drug to the patient.
Programmable control means can be provided for operating the drive motor continuously or at periodic intervals to obtain a closely controlled and accurate delivery of the drug over an extended period of time.
In this framework, the stopper typically has slow motion and is displaced on small distances in a given period of time.
As a consequence, it is necessary to ensure smooth gliding of the stopper within the container body to avoid any displacement by fits and starts.
In particular, the phenomenon known as “stick-slip effect” has to be avoided because it could lead to the delivery of an inaccurate dose of drug to the patient.
In order to improve gliding performance of the stopper, a lubricant coating may be applied onto the inner wall of the container body and/or onto the stopper.
Meanwhile, the stopper needs to provide a good sealing with respect to the container body in order to avoid leakage of drug from the container and to avoid any contamination of the drug from the outside of the container.
Traditional injections devices comprise a container made of glass or plastic and a stopper made of elastomeric material.
Due to the softness of the elastomer, the stopper itself is able to ensure the sealing with respect to the container body.
However, said softness may also be responsible for inaccurate dose delivery.
Indeed, when a pushing effort is applied to the stopper via the plunger rod, the stopper may first absorb the effort by its own compression, and may only begin gliding later on.
In such case, the dose delivered to the patient is different from the dose expected from the displacement of the plunger rod.
Moreover, in some cases, rubber and soft materials of the like may not be appropriate due to their incompatibility with the drug contained inside the container body of the injection device.
A more compatible material may thus be selected.
Therefore for the different above-mentioned reasons, it may be preferable to choose a rigid material for the stopper, e.g. polyethylene or the like.
Indeed, a stopper in this kind of material is not likely to deform under the pushing effort of the plunger rod and may thus allow improving the accuracy of the dose delivery.
In such case, it is necessary to provide at least a sealing ring around the stopper in order to ensure the sealing between the stopper and the container body, as such rigid materials usually lead to the leakage of the drug contained inside the container body.
The document US 2009/0326458 discloses a medical injection device that comprises a container and a rigid polyethylene stopper having at least one O-ring made of an elastomer, e.g. rubber or silicone, that is maintained in a groove of the stopper.
FIG. 1 illustrates a schematic sectional view of such stopper 2 having two peripheral grooves 21, 21′ and two O-rings 4, 4′ in order to be in gliding engagement within a container body 1, the inner wall 10 of the container body 1 being coated with a lubricant layer, e.g. a silicone-free lubricant.
However, this device can present several limitations.
First of all, the lubricant coating applied forms droplets that may be released in the drug 3.
Indeed, there is a strong concern about the presence of particles in the drugs, because they may alter the efficiency of the drug.
It is thus desired to limit the release of particles, in particular from the lubricant coating.
Further, due to their intrinsic properties, the silicone O-rings present certain permeability to moisture and air and do not provide either an efficient sealing of the injection device or a good drug isolation.
In addition, it is known that a back-and-forth movement of the stopper occurs under pump rod displacement, which can often be detrimental to the reactivity of the injection device and the accuracy of the dose delivered to the patient.